The number of details for each topic will vary depending on your main points. I would include comparisons (how they are similar) in the introduction and conclusion, but sections 2 and 3 and all those details will be stating and explaining how they are different.

There are two recognized patterns for writing comparison/contrast papers. One is casually referred to as "zig-zag,” but can be very confusing for the reader if you don’t use transitions effectively. The other is topic-by-topic (which is what I've outlined above) and is much easier for the reader to follow.